code: def test_builder_to_backend_power(self, compute_unit, backend):
      input_placeholders = {
          "a": mb.placeholder(shape=(1,)),
          "b": mb.placeholder(shape=(1,)),
      }
  
      def build(a, b):
          # Compute a^b
          def body(res, bx):
              return mb.mul(x=res, y=a), mb.add(x=bx, y=np.float32(1))
  
          def cond(res, bx):
              return mb.less(x=bx, y=b)
  
          res, ignored = mb.while_loop(
              _cond=cond, _body=body, loop_vars=([1.0], [0.0])
          )
          return res
  
      input_values = {
          "a": np.array([2], dtype=np.float32),
          "b": np.array([4], dtype=np.float32),
      }
  
      expected_output_types = [
          (1, types.fp32),
      ]
  
      expected_outputs = [
          np.array([16], dtype=np.float32),
      ]
      run_compare_builder(
          build,
          input_placeholders,
          input_values,
          expected_output_types,
          expected_outputs,
          compute_unit=compute_unit,
          backend=backend,
      )
  
imports: import itertools
  import numpy
  import pytest
  from coremltools.converters.mil.mil import Builder
  from coremltools.converters.mil.mil import get_new_symbol, types
  from coremltools.converters.mil.mil.ops.tests.iOS14 import backends
  from coremltools.converters.mil.mil.ops.tests.testing_utils import UNK_SYM, construct_inputs_from_placeholders, run_compare_builder
  from coremltools.converters.mil.testing_reqs import compute_units
  from coremltools.converters.mil.testing_utils import random_gen, ssa_fn
label_desc: Use image node_id 2 for calling the TestWhileLoop obj's underlying member method code with example usage: obj.test_builder_to_backend_power(compute_unit, backend) and returns: res, mb,  mb, mb